This invention relates to systems for joining two or more parallel pipes, cables or other elongate elements during offshore operations, for example in a ‘piggyback’ arrangement during pipelaying. The invention encompasses joining devices and apparatus and methods for fitting such joining devices to and between pipes, cables or other elongate elements.
It is often desirable to install two or more elongate elements along the same subsea route, such as a primary larger-diameter pipe for carrying hydrocarbons and a secondary smaller-diameter pipe for carrying water, gas or chemicals used to produce hydrocarbons.
Whilst pipes will be used as an example in this specification, an element need not be a pipe for carrying fluids but could instead be a cable for carrying power or data. A secondary element will usually be of much smaller diameter (typically <20 cm) than a primary element, but a difference in size between the elements is not essential to the invention in a broad sense.
Where elements such as pipes or cables are to follow the same route, it may be beneficial to install the elements simultaneously. This is commonly achieved by a piggyback technique where one or more secondary elements are attached by a succession of clamps to a primary element on a pipelay vessel, and the elements are then launched together in parallel toward the seabed.
Installation of a piggyback pipeline usually involves unspooling the secondary pipe on a pipelay vessel. The primary pipe may also be unspooled in a reel-lay arrangement although it could be fabricated on the pipelay vessel, for example in an S-lay operation.
A typical reel-lay vessel 10 shown schematically in FIG. 1 is fitted with a storage and deployment reel 12 for deploying a primary pipe 14 and has an adjustable lay ramp 16 that is capable of deploying a range of products at varying lay angles, which may be from circa 20° to 90° to the horizontal. The inclination of the lay ramp 16 is determined by the depth of water in which the pipeline is being laid and by the characteristics of the pipeline, such as its diameter and stiffness.
In downstream succession from the reel 12, the lay ramp 16 carries a guide chute 18 for guiding the primary pipe 14; a pipe straightener 20 for straightening the primary pipe 14; a track-type tensioner 22 for gripping the primary pipe 14 between articulated tracks; and a hold-off clamp 24 for clamping the primary pipe 14 whenever the tensioner 22 releases the primary pipe 14. A travelling clamp could be used instead of a track-type tensioner 22; references in this specification to a tensioner should be taken to include a travelling clamp unless the context demands otherwise.
As FIG. 2 shows, a piggyback reel 26 can be fitted to a vessel 10 for deploying a secondary element such as a secondary pipe 28 with the primary pipe 14 when operating in piggyback mode. In that mode, a piggyback chute 30 guides the secondary pipe 28 and the secondary pipe 28 is brought into alignment with the primary pipe 14, such that the secondary pipe 28 lies parallel to the primary pipe 14 downstream of the tensioner 22. The secondary pipe 28 then lies directly above the longitudinal centerline of the primary pipe 14 or, when the primary pipe 14 is vertical, directly aft of the longitudinal centerline of the primary pipe 14. The secondary pipe 28 is then ready to be clamped to the primary pipe 14 at work platforms in a shelter 32 on the lay ramp 16 between the tensioner 22 and the hold-off clamp 24.
In practice an additional straightener may be used for the secondary pipe 28 downstream of the piggyback chute 30 but this has been omitted from FIG. 2 for clarity. Also, the secondary pipe 28 may go through an additional tensioner but such a tensioner may not be required and has also been omitted for clarity.
In a prior art piggybacking arrangement, it is known for a secondary pipe 28 to be diverted entirely around the tensioner 22 before being aligned with the primary pipe. This makes it difficult to align the secondary pipe 28 without overbending it or requiring additional straightening, unless there is a substantial and disadvantageous gap under the tensioner 22. The heavy tensioner 22 should be mounted as low as possible on the lay ramp 16 to aid the stability of the vessel 10.
U.S. Pat. No. 5,975,802 to Willis (Assignee: Stolt Comex Seaway Ltd.) discloses a known piggyback arrangement in detail, including the relationship between the paths of a primary pipe and a secondary pipe as they pass over their respective chutes and are brought together for clamping. In the example shown in U.S. Pat. No. 5,975,802, the primary pipe is fabricated on board the pipelay vessel and the secondary pipe is unspooled from a reel, although it will be clear to the skilled reader that both pipes could be spooled with the addition of a storage and deployment reel for the primary pipe, as in FIG. 2. The content of U.S. Pat. No. 5,975,802 is incorporated herein by reference, as technical background to the present invention.
A known piggyback clamp 34 shown in FIG. 3 employs a tapered saddle-like block 36 of rubber or polyurethane between a primary pipe 14 and a secondary pipe 28. The block 36 has a concave undersurface shaped to fit the cross-sectional curvature of the primary pipe 14 and a hole for encircling and retaining the secondary pipe 28. The block 36 is in two parts that, when assembled together, define the hole and surround the secondary pipe 28.
In use, the two parts of the block 36 are assembled around the secondary pipe 28 to retain the secondary pipe 28 in the hole. The block 36 retaining the secondary pipe 28 is then attached to the primary pipe 14 by tensioned parallel circumferential straps 38 that encircle the primary pipe 14 and the block 36. The straps 38 keep the two parts of the block 36 together while holding the secondary pipe 28 parallel to and spaced slightly from the primary pipe 14.
The service demands on the clamp 34 are high. The block 36 and the straps 38 must survive the stresses of launching the pipeline from the pipelay vessel 10 to the seabed. The block 36 and the straps 38 may also need to survive the load of pulling the secondary pipe 28 off the piggyback reel 26 if no additional tensioner is used. Thereafter the block 36 and the straps 38 must continue to retain the secondary pipe 28 on the primary pipe 14 for the life of the pipeline, typically at least twenty years, without significant relative movement between the pipes 14, 28.
During piggyback operations on a pipelay vessel 10 such as that shown schematically in FIG. 2 or as described in detail in U.S. Pat. No. 5,975,802, manual intervention is required close to the pipes 14, 28 on the lay ramp 16 at a location downstream of the tensioner 22, to position, align and manually clamp the pipes 14, 28. In particular, a succession of clamps 34 must be assembled and fitted to the pipes 14, 28 by workers operating in a confined space on the lay ramp 16, which is steeply inclined and will pitch as the pipelay vessel 10 rides the waves. Considerations of safety and accuracy make it necessary to reduce the linear travel speed of the pipes 14, 28 with respect to the vessel 10 while the clamp installation process is carried out, or intermittently to stop the pipelay movement altogether.
Piggyback operations are therefore labour-intensive and inefficient, not just in labour costs but also in vessel time—which is typically worth circa US$300,000 per day. Pipelay rates in piggyback mode may be less than 500 m per hour, and possibly as little as 300-400 m per hour. This is less than half of the typical speed of reel-lay operations without piggybacking, and so approximately doubles vessel time on station and hence greatly increases vessel cost during pipelaying. It will also be apparent that if a pipelay vessel must be on station for say four days instead of two days, it is more likely to encounter weather conditions that will disrupt the pipelaying operation or force its temporary abandonment, again with a potentially great increase in time and cost.
If it would be possible to increase the speed of pipelaying in piggyback mode to approach the typical speed of pipelaying without piggybacking, the cost saving would be very substantial. Of course, it is essential for that saving to be achieved without compromising safety.
It is against this background that the present invention has been devised.